500 W. von Siemens on the General 



lead to the greatest contradictions and discontinuities. For 

 in the assumed original condition from which both Ferrel and 

 I start, when as yet no meridional currents existed, the air of 

 each latitude rotated with the velocity of the ground upon 

 which it rested, and its velocity therefore diminished with the 

 cosine of the latitude. Now, according to Ferrel's views, 

 this relation must not only have been reversed with the 

 appearance of meridional currents, but instead of a diminution 

 an increase of the velocity of the air must have occurred in a 

 yet higher degree, if the moment of inertia of the air is sup- 

 posed to remain constant. But why it must remain constant, 

 and what forces could bring about so considerable an increase 

 of the vis viva of the rotating air are equally incomprehensible *. 

 i^nother of FerrePs assumptions with which I cannot agree 

 is that the uppermost strata of air can slide down inclined 

 surfaces of equal air-pressure, for in these there is as little 

 tendency to tangential displacement as on a level surface. 

 That such a displacement could not in general take place 

 follows at once from the consideration, that even if a current 

 of air did descend, a difference of pressure would arise, with 

 a consequent disturbance of the balance of pressure, and the 

 immediate production of a back current. It hence follows 

 that meridional currents of air cannot be caused by the steady 

 increase in the temperature of the air from the poles to the 

 equator which (disturbances excepted) is found to exist, and 

 to this Dove agrees. Surfaces of equal pressure exist at all 

 levels throughout such an unequally heated atmosphere which 

 reach from the equator to the poles, and along which no 

 spontaneous motion of the air can take place. Notwithstand- 

 ing the greater rarefaction or loosening of the air through 

 the heat of the torrid zone, the air would remain at rest, if 

 the indifferent equilibrium were not disturbed in some 

 portion of it. The real condition of equilibrium and of 



* I must emphatically protest against Dr. Sprung's statement, u that 

 there is the same (certainly very excusable) error contained in my assump- 

 tion of a constant velocity of rotation of the air, as in the conception of 

 Hadley and Dove on the influence of the earth's rotation on the motion 

 of the air." Dr. Sprung is not entitled to bring forward von Helmholtz's 

 communication "On Atmospheric Motions" in support of this opinion, 

 for in this mathematical inquiry von Helmholtz has considered a " hypo- 

 thetical case." He says, " If we assume a rotating ring of air — the axis 

 of which coincides with the earth's axis — to he displaced at one time to 

 the north and at another to the south by the pressure of similar neigh- 

 bouring rings, the moment of inertia must remain constant, in accordance 

 with the well-known general mechanical principle." This is no doubt 

 correct, for in the assumed case the pressure of the neighbouring rings 

 performs the work of acceleration ; but the point in question is just 

 whether there are forces available to exert this displacing pressure. 



